Thin phosphorus nitride film as an n-type doping source used in laser doping technology

ABSTRACT

An improved method and system for laser doping a semiconductor material is described. In the invention, phosphorous nitride is used as a dopant source. The phosphorous nitride is brought into close proximity with a region of the semiconductor to be doped. A pulse of laser light decomposes the phosphorous nitride and briefly melts the region of semiconductor to be doped to allow incorporation of dopant atoms from the phosphorous nitride into the semiconductor.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional of application Ser. No. 09/473,576; filedDec. 28, 1999.

FIELD OF THE INVENTION

[0002] The invention relates to an improved method of laser doping asemiconductor material. In particular, the invention describes animproved doping source for use in laser doping.

BACKGROUND

[0003] In recent years, laser doping has become a popular method forfabricating large area electronics. The low processing temperatures usedin a laser doping process makes the process suitable for dopingsemiconductor layers on substrates that have low temperature tolerances.Examples of substrates with low temperature tolerances include plasticand glass.

[0004] A second reason for the popularity of laser doping is theavailability of techniques to make self-aligned amorphous silicon ThinFilm Transistors (TFT). Self aligned techniques for forming TFTs utilizethe gate electrode as a mask, thereby minimizing alignment problems whenforming a passivation island over the gate electrode. Detailed methodsfor forming such self aligned structure are described in an article byP. Mei, G. B. Anderson, J. B. Boyce, D. K. Fork, and R. Lujan entitledThin Film Transistor Technologies III published in the ElectrochemicalSoc. Proc. PV 96-23., p. 51 (1997). U.S. patent application Ser. No.D/97343 entitled “Method of manufacturing a Thin Film Transistor withreduced parasitic capacitance and reduced feed-through voltage” by P.Mei, R. Lujan, J. B. Boyce, C. Chua and M. Hack, also describesfabricating a TFT structure using a self alignment process and is herebyincorporated by reference.

[0005] During laser doping, a laser pulse briefly melts a surface layerin a doping region of a semiconductor. While the doping region is in amolten state, a dopant source is introduced near a surface of the dopingregion. The dopant source provides dopant atoms that are distributedthrough the surface layer. After the doping region solidifies, thedopants are available for electrical activation.

[0006] Typical dopant sources used to provide dopant atoms include: (1)gas dopant sources such as phosphorus fluoride (PF₅) and boron fluoride(BF₃), (2) spin-on dopants such as phosphorous doped silica solution,and (3) phosphor-silicon alloys which may be ablated from anothersubstrate or deposited directly on the doping region.

[0007] Each of the above described dopant sources has a correspondingdisadvantage. For example, gas dopant sources are difficult to controland require the use of specialized equipment to prevent unevendistribution of dopants. Furthermore, both gas and spin-on depositiontechniques have low doping efficiencies. A low doping efficiency makesit difficult to fabricate a heavily doped material. Phosphor-siliconalloy materials are difficult to deposit and unstable when exposed tomoisture in air. In particular, moisture in the air decomposes thephosphor-silicon alloy. After decomposition, the moisture mayredistribute the dopant atoms. Dopant atom redistribution produces highdefect rates in devices formed from the doped semiconductor.

[0008] Ion implantation provides an alternate method of distributingdopant atoms in a semiconductor. However, when used in large areaprocesses, ion implantation requires expensive specialized equipment.

[0009] Thus an improved method of laser doping is needed.

SUMMARY OF THE INVENTION

[0010] Although laser doping has become a popular method of fabricatinglarge area electronic devices, current laser dopant sources aredifficult to control, require expensive equipment, or are unstable whenexposed to moisture in the air. In order to avoid these problems, theinvention describes a technique for using a thin film of dopantcontaining compound (DCC) as a laser doping source. The dopantcontaining compound (DCC) can be deposited by standard equipment and isstable in air. An example of a dopant containing compound is phosphorousnitride.

[0011] In one embodiment of the invention, standard plasma enhancedchemical vapor deposition (PECVD) equipment is used to deposit a thinphosphorous nitride film over the dopant region. Alternatively, atransparent dopant plate may be used to position the phosphorous nitridenear the dopant region in a laser ablation process. In laser ablation, alaser decomposes the phosphorous nitride. The laser also briefly meltsthe semiconductor in the dopant region to allow incorporation of dopantatoms into the doping region. After the doping process, remainingphosphor nitride may be removed using a plasma containing small amountsof CF4 diluted in oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 illustrates a sequence of operations used to fabricate aThin Film Transistor (TFT) structure using one embodiment of theinvention.

[0013]FIG. 2. is a graph illustrating the resistivity of laser dopedamorphous silicon as a function of changing laser energies and dopantsources.

[0014]FIG. 3 illustrates a system that uses laser ablation to providedopant atoms from a film of phosphorous nitride to dope a semiconductorsample.

[0015]FIG. 4 is a plot illustrating the TFT transfer characteristic of adevice fabricated using a phosphorous nitride dopant.

DETAILED DESCRIPTION

[0016]FIG. 1 illustrates the fabrication of a thin film transistor (TFT)by laser doping portions of the TFT with a phosphorous nitride thinfilm. Although the structure described is a TFT structure, it isunderstood that the described doping techniques are applicable to alarge range of devices including other semiconductor active and passivedevices such as resistors and transistors used in integrated circuits.

[0017] In FIG. 1, a metal gate 104 is deposited and patterned over aglass substrate 108. A deposition techniques, such as plasma enhancedchemical vapor deposition (PECVD) is used to deposit a plurality oflayers over the gate 104 and substrate 108. In the embodimentillustrated, the layers include a nitride layer 112, an amorphoussilicon layer 116 and a top passivation layer 120. In one embodiment ofthe invention, top passivation layer 120 is a multilayer structureformed from alternating nitride and oxide multilayers. A backsideexposure that uses metal gate 104 as a mask may be used to form anisland 124 from the top passivation layer 120. Using a backside exposuresimplifies the alignment of island 124 with gate 104. The proceduresused to form and pattern the gate metal, to deposit the nitride,amorphous silicon and passivation layers and to pattern the passivationlayer are described in the previously referenced patent applicationentitled “Method of Manufacturing a Thin Film Transistor with ReducedParasitic Capacitance and Reduced Feed-Through Voltage”.

[0018] After formation of island 124, a doping region of amorphoussilicon is doped. In the illustrated embodiment, doping regions 129 arepositioned on both sides of island 124. To dope the amorphous silicon, aDCC compound that includes at least one element from either from columnIII or column V of the periodic table is brought in close proximity tothe amorphous silicon. Typical compounds may include the column III orcolumn V element chemically bonded to form a nitride or oxide. As usedherein, a “compound” is defined as a combination of two or more elementsthat form a chemical bond. Thus a “compound” does not include mixturesor alloys in which the elements are not chemically bonded together. Forexample, water is defined as a compound whereas a mixture of hydrogenand oxygen is for purposes of this patent not considered a compound.

[0019] One example of a compound that has been found to be suitable as adoping source is phosphorous nitride. The chemical bond between phosphorand nitride in the phosphor nitride compound makes the compound stablein air; phosphor itself is unstable in air.

[0020] A first method of forming phosphorous nitride is to evaporatephosphor in a N₂ plasma. The evaporated phosphor deposits over thesubstrate as a phosphorous nitride film. Alternatively, a pulsedischarge of a phosphorus powder covered electrode in a N₂ ambient gasmay also be used to form a phosphorous nitride compound. Both techniquesof generating a phosphorous nitride film are described in an articleentitled “Solid Planar Diffusion Sources Based on Phosphorus NitridePrepared by a One-stage Process in a Pulse Discharge” by M. Raicis andL. Raicis in Surface and Coatings Technology, Vol. 78, p. 238 (1996).

[0021] An alternate method of bringing a DCC, including an element fromcolumn 3 or column 5 of the periodic table, into close proximity toamorphous silicon uses commercially available PECVD equipment. In thefollowing description, example parameters used to achieve PECVDdeposition of a phosphorous nitride film 130 will be provided. In theexample, a processing gas of one sccm (Standard cubic centimeter perminute) PH₅ and 100 sccm NH₃ is formed. Ten watts of 13.5 MHz radiofrequency power in a 400 mTorr ambient pressure converts the processinggas into a plasma. Exposing a substrate, such as glass substrate 108 at250 degrees Centigrade, to the plasma results in a phosphorous nitridethin film with a reflection index of approximately 1.85. Using theprovided example parameters results in a deposition rate ofapproximately 50 Angstroms per minute.

[0022] After deposition, a laser beam 132 irradiates the phosphorousnitride thin film to laser dope select regions of amorphous silicon 116.In one embodiment, laser beam 132 originates from an excimer laser witha high fluence laser beam having a wavelength of 308 nm. The laser beambriefly melts the amorphous silicon and decomposes the phosphorous thinfilm 128. The decomposing phosphorous thin film introduces phosphoratoms into the molten layer of amorphous silicon.

[0023]FIG. 2 is a graph that illustrates the sheet resistance of a dopedamorphous silicon layer as a function of laser energy density used inlaser doping. Each curve of the graph corresponds to a different dopantsource. Curve 204 corresponds to a phosphorous nitride dopant source,curve 208 corresponds to a Psi alloy as a dopant source, curve 212corresponds to a P+ implantation at a dosage of 3×10¹⁵ cm⁻² and curve216 corresponds to a P+ implantation at a dosage of 1×10¹⁵ cm⁻². Themeasurements were taken on a structure including a nitride and a 500angstrom thick amorphous silicon layer deposited on a glass substrate.

[0024] To form the structure from which measurements were taken, 100Angstroms of phosphorous nitride film was deposited over the amorphoussilicon using the previously described example PECVD process. Pulses oflaser light from a XeCl excimer laser decomposes the phosphorousnitride.

[0025] After exposure to the excimer laser, a four point probe was usedto measure the resistivity of the resulting doped amorphous siliconstructure. At each laser energy density value plotted along a horizontalaxis of FIG. 2, the phosphorous nitride dopant source plotted as curve204, produced the lowest resistance doped amorphous silicon. A lowresistance demonstrates the high doping efficiency of the phosphorousnitride by indicating that almost all of the phosphorous nitride filmdecomposes under the laser exposure.

[0026] Returning to FIG. 1, after exposure of portions of thephosphorous nitride thin film to laser radiation, areas unexposed tolaser radiation may remain covered by the phosphorous nitride film. Inparticular, during fabrication of laser-doped, self-aligned TFTstructures, portions of phosphorous nitride films 128 deposited overpassivation island 124 may not absorb or receive sufficient exposure tolaser energy and thus may not be fully decomposed. Unexposed phosphorousnitride film has a very high resistivity. Therefore, in some TFTembodiments, the unexposed phosphorous nitride remains in the devicewithout disturbing the self-aligned TFT operation. The high resistivityof phosphorus nitride allows its use as a gate dielectric material forInP MISFETS as described in an article entitled “Enhancement mode InPMISFET's with sulfide passivation and photo-CVD grown P3N5 gateinsulators” by Y. Jeong, S. Jo, B. Lee and T. Sugano in the IEEEElectron Device Letters, Vol 16, Page 109 (1995).

[0027] When complete removal of phosphorous nitride is desired, a plasmacleaning process may be implemented. In one implementation of such aplasma cleaning process, a small amount of CF4, typically 1 to 2 percentin O₂, is used to remove the undesirable phosphorous nitride.

[0028] After removal of undesired portions of phosphorous nitride thinfilm 128, metal source contact 136 and metal drain contact 140 areformed over the doped areas of amorphous silicon layer 116. To completethe TFT structure, a second passivation island 144 of SiO₂ may be formedbetween metal source contact 136 and metal drain contact 140.

[0029] A typical transfer characteristic curve of a TFT fabricated bythe process described above is shown in FIG. 4. FIG. 4 plots anormalized drain current as a function of an applied gate voltage. Theperformance is comparable to devices formed using dopant sources thatare unstable in air such as structures described in the previouslyreferenced patent by Mei et. al.

[0030] Although the prior description has described using a PECVDtechnique to deposit the phosphorous nitride directly on a device forlaser doping, it should be recognized that other techniques may be usedto bring phosphorous nitride in close proximity to the device to bedoped. For example, FIG. 3 shows a system 300 for depositing phosphorousnitride using laser ablation. In FIG. 3, phosphorous nitride film 302 isdeposited over a surface of a transparent source plate 304. Thetransparent source plate is positioned in close proximity to thesemiconductor sample 306 with the phosphorous nitride film coatedsurface facing semiconductor sample 306. Gap 312 that separates thephosphorous nitride film 302 and semiconductor sample 306 is typicallyseveral microns, although the gap may range from zero to severalmillimeters. The height of spacers 308 determines the size of the gap.The smaller the gap between phosphorous nitride film 302 andsemiconductor sample 306, the higher the dopant efficiency.

[0031] After phosphorous nitride film 302 is properly positioned overthe semiconductor to be doped, a laser such as an excimer laser, outputsa laser beam that propagates through source plate 304 and irradiates aselect area of phosphorous nitride film 302. The phosphorous nitridefilm absorbs a portion of the laser energy. When the phosphorous nitridefilm absorption rate is too low, a thin layer of opaque material, suchas a-Si, may be inserted between plate 304 and phosphorous nitride film302 to enhance the efficiency of laser energy absorption. Absorbedenergy from the laser pulse ablates phosphorous nitride film 302 causingphosphorous nitride film 302 to release energetic dopants into the gapbetween film 302 and semiconductor sample 306.

[0032] In addition to ablating phosphorous nitride film 302, the laserenergy also melts a surface region of semiconductor sample 306. Thedepth of the melted region depends upon the laser energy and the pulsewidth (duration) as well as the thermal transport properties of thesemiconductor (typically amorphous silicon) sample 306. A typical laserpulse width is approximately 50 nanoseconds. After receiving the laserpulse, the semiconductor remains in a molten state for a short timeinterval allowing semiconductor sample 306 to incorporate some of thedopant atoms released in the gap during the laser ablation process. Theabsorbed dopants are activated when semiconductor sample 306 solidifies.A more detailed description of using laser ablation is described in U.S.Pat. No. 5,871,826 (the '826 reference) entitled Proximity Laser DopingTechnique for Electronic Materials by inventors Ping Mei, Rene A. Lujanand James B. Boyce which is hereby incorporated by reference.

[0033] In the '826 reference, a PSi doping material is the suggesteddoping material. However, PSi doping material tends to break down whenexposed to moisture in air. The stability in air of DCC compounds, suchas phosphorous nitride, allows the dopant source plate to be preparedoff line in a batch process and stored until needed for use.

[0034] While the invention has been described in terms of a number ofspecific embodiments, it will be evident to those skilled in the artthat many alternative, modifications, and variations are within thescope of the teachings contained herein. For example, variations inparameters may occur, such as the type of DCC compound, the method ofdeposition of the DCC compound, the type of laser used and the devicebeing formed. Examples of other types of devices that may be formedusing laser doping of phosphorous nitride include the fabrication ofshallow junctions in Very Large Scale Integrated Circuits (VLSIs).Accordingly, the present invention should not be limited by theembodiments used to exemplify it, but rather should be considered to bewithin the spirit and scope of the following claims, and equivalentsthereto, including all such alternatives, modifications, and variations.

1. A method of doping a semiconductor comprising the operations of:placing a layer of a dopant containing compound that is stable in air inclose proximity to a semiconductor surface; and directing a laser todope the semiconductor with atoms from the layer of dopant containingcompound.
 2. The method of claim 1 wherein the dopant containingcompound is formed from a nitride and at least one element from a groupof column three or column five of the periodic table.
 3. The method ofclaim 1 wherein the dopant containing compound is phosphorous nitride.4. The method of claim 1 wherein the operation of placing the layer ofdopant containing compound in close proximity to the semiconductorsurface includes: depositing the dopant containing compound on thesemiconductor surface using plasma enhanced chemical vapor deposition.5. The method of claim 3 wherein the operation of placing the layer ofdopant containing compound in close proximity to the semiconductorsurface includes: depositing the dopant containing compound on atransparent surface; and positioning the dopant containing compound overthe semiconductor surface to allow a laser beam from the laser to passthrough the transparent surface and cause atoms from the dopantcontaining compound to dope the semiconductor.
 6. The method of claim 3wherein a laser beam from the laser: decomposes the phosphorous nitride;and melts the semiconductor.
 7. The method of claim 3 further comprisingthe operation of: removing excess phosphorous nitride by using a plasmaof CF4 in oxygen.
 8. The method of claim 1 further comprising theoperation of: forming the semiconductor on a glass substrate prior toexposure to the laser.
 9. A method of forming a thin film transistorstructure comprising the operation of: depositing a gate on a substrate;forming an amorphous silicon layer over the gate and the substrate;forming a passivation layer over the amorphous silicon layer; creatingan island from the passivation layer over the gate; positioning dopantcontaining compound over the amorphous silicon layer; exposing thedopant containing compound to a laser beam to dope the amorphous siliconwith atoms from the dopant containing compound to create doped regionsof amorphous silicon.
 10. The method of claim 9 further comprising:depositing and patterning metal contacts for a source and drain over thedoped regions of amorphous silicon.
 11. The method of claim 9 whereinthe creating of the island from the passivation layer utilizes abackside exposure in which the gate serves as a mask.
 12. The method ofclaim 9 further comprising: removing excess dopant containing compoundusing plasma of dilute CF4 in oxygen.
 13. The method of claim 9 whereinthe dopant containing compound is phosphorous nitride.
 14. The method ofclaim 13 wherein the positioning of the phosphorous nitride is doneusing a plasma enhanced chemical vapor deposition process.
 15. Themethod of claim 13 wherein the positioning of the phosphorous nitride isdone by positioning a transparent source plate, the transparent sourceplate includes a film of phosphorous nitride.
 16. The method of claim 13wherein a gap separates the phosphorous nitride film from the amorphoussilicon, the doping of the amorphous silicon occurs through phosphor andnitrogen atoms that cross the gap.
 17. A method of doping asemiconductor material with a donor material comprising the operationsof: providing a source plate; providing a layer of phosphorous nitrideon one side of the source plate; placing the source place in closeproximity with the semiconductor material with the one side facing thesemiconductor material; irradiating the phosphorous nitride with a lasersource passing through the source plate, the irradiation resulting inablating the phosphorous nitride layer and melting a region of thesemiconductor material; and introducing atoms from the phosphorousnitride into the region of the semiconductor material.
 18. The method ofclaim 17 further comprising the operation of: providing an absorptionlayer between the source plate and the phosphorous nitride layer. 19.The method of claim 17 wherein the semiconductor material is amorphoussilicon; the region of the amorphous silicon having a sheet resistivitybelow 1000 ohms per square.
 20. A structure to be used in the formationof a thin film transistor comprising: a substrate; a gate deposited overthe substrate; a semiconductor layer formed over the substrate and thegate; and a phosphorous nitride film formed adjacent to thesemiconductor layer, the phosphorous nitride film decomposed and used todope the semiconductor layer.